I have an SQL query which links 3 tables using UNION:

$sql ="(SELECT Drive.DriveID,Ram.Memory from Drive,Ram where Drive.DriveID = Ram.RamID) UNION
(SELECT Drive.DriveID,External.Memory from Drive, External where Drive.DriveID开发者_如何转开发 = External.ExtID)";

Suppose I want to get Ram.Name as well. How do I do this? If I use Ram.Name in the first SELECT statement it would not produce the correct result.

Any method for tackling this? I want to do it using UNION.

how about:

SELECT Drive.DriveID,Ram.Memory,Ram.Name 
FROM Drive,Ram WHERE Drive.DriveID = Ram.RamID
UNION
SELECT Drive.DriveID,External.Memory,NULL 
FROM Drive, External WHERE Drive.DriveID = External.ExtID

this will have NULL in the fourth column for all of the results of the second select